Boeing/Honeywell FMC POS REF "UPDATE ARM"

[RandomPerson8008]

Hello,

Can anyone explain in what situation you would use the "UPDATE ARM" selection on FMC POS REF Page 2? I'm guessing that it would be used in a situation where you were in an area with no GPS or Radio reception and wanted to update the position, for instance just prior to coast-in? Wouldn't the FMC automatically do this, though, as soon as radio or GPS reception was again available? Is it a defense against GPS spoofing?

Thanks in advance. I wasn't able to decipher the meaning of the selection from the FCOM.

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/417x556/image1_6__107df64d6d5179206dc5ebb6560864d112a3be44.png
https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/417x556/image0_11__f437faa83d37d04133bd2270f9fa4b21895ce7f0.png

[EXDAC]

Hello,

Can anyone explain in what situation you would use the "UPDATE ARM" selection on FMC POS REF Page 2?

I didn't spend much time with the "brown" FMC but my understanding of the description is that "NOW" only appears at 2R, 3R, or 4R, if 1R was pressed and shows "ARMED". However, the example display shows "NOW" at 4R but 1R displays "ARM" not "ARMED".

The display does not seem to be consistent with the text but I don't think you can "update now" without having "armed" update with 1R.

I'm sure someone with more recent experience with this system will correct me if I'm wrong.

[pilot-737]

I will try to interpret the given data as I’m not flying Boeings anymore.

In this menu you can update (or move to if you prefer) your present FMC position to one of the calculated positions by
1) IRS
2) GPS
3 Radio nav calculated position.

As this is an action which can have significant implications, a dual action is required :
1) Arm the function
2) Chose which source you prefer by pressing “now”

All sources may not be available and all possible cases are depicted

1) IRS Position is not available (“now” is missing) because the ANP is out of limits : 4.0 while RNP is 0.5
2) GPS position update is not available (“Now” is missing), as it seems that is already the source providing the data (FMC coordinates perfectly match to GPS coordinates).
3) Nav aid position is available and within RNP limits thus “now” is displayed.

Af far as I understand, with this menu, you can intervene to the automatic priority (if GPS not available go to Navaid. If Navaid not available go to IRS), and this action will be permanent(until you change again the source to GPS for example).

Now a possible scenario to use this menu:

You are aware by NOTAM that you will cross an area, in which you may receive incorrect GPS information.
In this case you deselect GPS as the primary source of FMC position and you select the Navaid position.
When out of the area, you can switch back to GPS position.

[back to Boeing]

Regularly flying through turkey where gps jamming is a constant feature we’ve had no info about this from our technical pilots. All we’ve been told is to turn on DME DME updating. So
If anyone has any more info as to when you’d use this function I’d be interested.

[8che]

"" Can anyone explain in what situation you would use the "UPDATE ARM" selection on FMC POS REF Page 2? I'm guessing that it would be used in a situation where you were in an area with no GPS or Radio reception and wanted to update the position, for instance just prior to coast-in? Wouldn't the FMC automatically do this, though, as soon as radio or GPS reception was again available? Is it a defense against GPS spoofing? ""

Yes when the aircraft tells you too !

B787 = EICAS FMC VERIFY POSITION , run the check list its all there..
or
B777 = FMC Alert Message VERIFY POSITION (primary /secondary FMC position differs by more than the greater of RNP or 0.4 NM for 5 seconds or FMC position differs with navaid used by greater than 12nm for 5 seconds.)